Many pourable food products, such as fruit juice, UHT milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example of such a package is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is formed by folding and sealing laminated strip packaging material having a multilayer structure comprising a layer of fibrous material, e.g. paper, covered on both sides with layers of heat-seal plastic material, e.g. polyethylene.
In the case of aseptic packages of long-storage products such as UHT milk, the packaging material has a layer of barrier material, e.g. an aluminium sheet, which is superimposed on a layer of heat-seal plastic material and is in turn covered with another layer of heat-seal plastic material eventually defining the inner face of the package contacting the food product.
As is known, such packages are made on fully automatic packaging units, on which a continuous tube is formed from the web-fed packaging material; the web of packaging material is sterilized on the packaging unit itself, e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution, which, after sterilization, is removed, e.g. vaporized by heating, from the surfaces of the packaging material; and the web of packaging material so sterilized is maintained in a closed sterile environment, and is folded and sealed longitudinally to form a vertical tube.
The tube is fed continuously in a first vertical direction, is filled with the sterilized or sterile-processed food product, and is gripped at equally spaced cross sections by two pairs of jaws. More specifically, the pairs of jaws act cyclically and successively on the tube to heat seal the packaging material of the tube and form a continuous strip of pillow packs connected to one another by respective transverse sealing bands, i.e. extending in a second direction perpendicular to said first direction.
The pillow packs are separated by cutting the relative transverse sealing bands, and are then conveyed to a final folding station where they are folded mechanically into the finished parallelepiped shape.
The portion of the tube gripped between each pair of jaws is heat sealed by heating means carried on one of the jaws and for locally melting the two layers of heat-seal plastic material gripped firmly between the jaws.
More specifically, when the layer of barrier material is defined by a sheet of electrically conductive material, e.g. aluminium, the packaging material is normally sealed using a so-called induction heat-seal process, in which, when the tube is gripped by the jaws, eddy current is induced in the aluminium sheet to heat the aluminium sheet locally and so locally melt the heat-seal plastic material.
More specifically, in induction heat sealing, the heating means substantially comprise an inductor, which is carried by one of the two jaws, known as the sealing jaw, is supplied by a high-frequency current generator, and is substantially defined by one or more induction bars made of electrically conductive material, extending parallel to the second direction, and interacting with the tube material to induce eddy current in and heat the material to the required sealing temperature. The other jaw, known as a counterjaw, has pressure pads of elastomeric material, which cooperate with the induction bars to heat seal the tube along a respective transverse sealing band.
At the end of the sealing operation, a cutting member, carried by one of the two jaws, normally the counterjaw, and interacting with the tube of packaging material, is activated to cut the tube along the centerline of the transverse sealing band and so cut a pillow pack off the bottom end of the tube of packaging material. The bottom end is therefore sealed transversely, and the jaws, on reaching the bottom dead center position, are opened to avoid interfering with the top portion of the tube. At the same time, the other pair of jaws, operated in the same way, moves down from the top dead center position and repeats the gripping/forming, sealing and cutting operations described above.
From analysis of the packaging material during the heat-seal operation, the eddy current induced in the cross section of the tube of packaging material gripped between a respective pair of jaws has been found to travel along an endless path, which is linear on the two longitudinal sides of the inductor-tube interaction portion, i.e. along the sides parallel to the second direction, and is roughly semicircular close to the edges of the cross section. In other words, the current travels linearly in opposite directions along the two longitudinal sides of the inductor-tube interaction portion, and, close to the edges of the cross section gripped between the jaws, deflects towards the center of the cross section (“bending-off effect”), so that the transverse sealing band is narrower at the ends than at the center portion, i.e. the portion intersecting the longitudinal seal initially formed to produce the tube of packaging material. Moreover, when packaging pourable food products containing small solid particles (such as seeds in tomato products) which may get trapped between the unsealed portions of the two contacting sheets of the packaging material, as wide a transverse sealing band as possible is desirable to reduce the likelihood of channeling through the sealed portion.
To eliminate the above drawbacks, Patent Application EP0992431, filed by the present Applicant, proposes that each cross section of the tube of packaging material be cut before being sealed.
As described in the above European patent, inverting the cutting and sealing operations produces a variation in the path of the eddy currents induced in the cross sections of the tube of packaging material. That is, the parting line produced by the cutting member in the tube of packaging material interrupts the electric continuity of the aluminium sheet, so that the eddy currents induced by the induction bars in the packaging material are confined to opposite sides of the parting line. In other words, the eddy current induced in the packaging material by the induction bars on one side of the parting line tends towards the induction bars on the opposite side of the parting line, but, owing to the interruption in the packaging material, is forced to complete a closed path on the same side of the parting line.
The sealing area on both sides of the parting line is therefore more or less constant, by drastically reducing the bending-off effect of the eddy current close to the edges of the cross section of the tube gripped between the jaws.
Number 50 in FIG. 6 indicates as a whole one example of a known sealing jaw which may be used in particular on packaging units in which the cross sections of the tube of packaging material are cut before being induction sealed.
Jaw 50 has a plane M of symmetry perpendicular to the traveling direction of the tube of packaging material, and comprises two induction elements 51, 52 housed inside respective face seats on jaw 50 and interacting with the packaging material via respective pairs of active surfaces 53, 54.
More specifically, induction element 51 is U-shaped, has a substantially annular cross section, and externally defines the two active surfaces 53, which are located symmetrically on opposite sides of plane M. Induction element 52 is defined by a straight bar having a U-shaped cross section, housed along the middle of jaw 50, and defining the two active surfaces 54, which are located on opposite sides of plane M and between active surfaces 53.
Active surfaces 53, 54 all have continuous or segmented longitudinal projections projecting towards the packaging material.
When using sealing jaw 50, the eddy current induced in the packaging material has been found to travel along endless, substantially symmetrical paths on opposite sides of the parting line produced by the cutting member. More specifically, on each side of the parting line, the eddy current travels linearly along each active surface, and deflects minimally close to the edges of the packaging material; which deflection only affects a small part of the sealing portion and is therefore negligible.
While providing for wider transverse sealing bands than those obtainable without inverting the cutting and sealing operations, sealing jaw 50 described above has several minor drawbacks preventing its many advantages from being used to the full.
In particular, FIG. 7 shows a graph of the power induced (continuous line) and the temperature (dash line) in the aluminium layer of the cross section of the tube of packaging material interacting with jaw 50 as a function of the distance from plane M or, equally, from the parting line of the cross section.
The FIG. 7 graph refers to only one of the two halves into which the cross section of the tube of packaging material interacting with jaw 50 is ideally divided by plane M, it being understood that the induced power and temperature curves in the other half of the cross section are perfectly symmetrical with those shown with respect to plane M.
As shown in the FIG. 7 graph, given the geometry of sealing jaw 50, and in particular of active surfaces 53, 54 of induction elements 51, 52, the power induced in the aluminium layer reaches a peak P1* at plane M and a peak P2* at each active surface 53, and assumes a minimum value between each active surface 54 and the adjacent active surface 53. More specifically, peak P2* is much lower than peak P1*.
Temperature distribution along the aluminium layer of the packaging material is a direct result of the induced power distribution produced by the dimensional ratios of active surfaces 53, 54 of induction elements 51, 52.
More specifically, moving in the traveling direction of the packaging material away from plane M or, equally, from the parting line produced in the cross section by the cutting member, the temperature in the aluminium layer of the cross section of the tube of packaging material interacting with jaw 50 falls sharply (by about 40%).
The above temperature distribution results in overheating of the layers of heat-seal plastic material at the mid longitudinal portion of the transverse sealing band, so that the molten material gripped between the jaws tends to flow outwards of the sealing band, thus impairing the quality of the seal.
To make up for the low power induced in the eddy current deflection regions, which results in reduced heating of the packaging material, as deducible from FIGS. 6 and 7 combined, sealing jaw 50 must be equipped, at the current deflecting regions, with inserts made of magnetic-flux-concentrating material, such as ferrite-containing composite material; and similar inserts must be provided on jaw 50 at the intersection between the transverse sealing band and the longitudinal seal on the tube of packaging material. In fact, at said intersection, where the packaging material for sealing is thicker owing to the presence of three superimposed portions of material, the sealing quality obtainable, without the inserts, in the lesser heated portion of the packaging material is far from satisfactory.